The present invention relates to a rubber composition for a tire tread which comprises a novel modified styrene-butadiene copolymer rubber, and specifically to a rubber composition for a tire tread which has an excellent processability, and is remarkably improved in respect of head build-up without detriment to high grip and abrasion resistance.
With a recent trend of upgrading of the performance of vehicles and spread of the expressway network, a demand for a car tire providing a running stability has been greatly increased. Thus, in the market, a so-called high performance tire having a high grip and an excellent high speed durability has been increasingly demanded, while in the field of racing tires used under severer conditions in races, rallies, etc., there has been an increasing demand for a tire having a tread portion comprising an excellent rubber composition capable of showing a high grip even under extreme road surface conditions and having an excellent blowout resistance.
In the conventional high performance or racing tire, a high grip has been generally secured by using a rubber composition of the tread portion which comprises as the raw material rubber a styrene-butadiene copolymer rubber having a high styrene unit content (hereinafter referred to briefly as "high-styrene SBR"), which is manufactured by the emulsion polymerization method, carbon black having a relatively small particle size, and a large amount of a softener represented by aromatic oil.
However, since such a rubber composition involves a defect in processability of too strongly adhering to the wall, rotor drop door, or roll of a mixer (e.g., a Banbury mixer), a serious difficulty has been experienced in obtaining a practical rubber composition. In addition, such a rubber composition involved another defect in performance of being liable to blow out due to ready heat build-up thereof, thus leading to a poor high speed durability when it is used in a tire tread.
As a result of intensive investigations with a view to solving the above-mentioned problems, the inventors of the present invention have previously found that a composition comprising as a rubber component a high-styrene SBR having specified molecular weight and composition distributions, which is prepared by the solution polymerization method, can remarkably improve the processability, and has low heat build-up and an improved abrasion resistance without detriment to the high grip performance (Japanese Patent Application No. 37,617/1985).
As a result of further investigations, the inventors of the present invention have found that the use of a high-styrene SBR prepared by the solution polymerization and having a specified atomic group introduced into a molecular terminal or chain thereof can provide a rubber composition which has attained a further dramatic decrease in the heat build-up and a remarkable improvement in the blowout resistance while keeping the excellent processability and the high grip performance. This has led to completion of the present invention.